1. Technical Field
The disclosure relates to a control system of a power transmission system that is located between a motive power source and drive wheels, and includes a fluid coupling, and an engagement device that can directly couple the input shaft side and output shaft side of the fluid coupling.
2. Description of Related Art
When a vehicle passes along an undulating road with a markedly uneven road surface, a power transmission system for transmitting driving force from a motive power source, such as an engine and a motor, to drive wheels vibrates along with vertical vibration of the wheels. Further, resonance is generated in a particular portion of the power transmission system, depending on the uneven condition of the road surface and the traveling speed of the vehicle. Under this situation, where the power transmission system includes a hydraulically-operated friction engagement element, the friction engagement element that is hydraulically placed in an engaged state may slip due to variation of transmission torque caused by the vibration, and its durability may deteriorate.
Thus, when resonance is generated in the power transmission system having a friction engagement element or elements for changing the speed ratio, a hydraulic pressure supplied to the friction engagement element that is in an engaged state is further increased, so as to further increase the engaging force of the friction engagement element, according to a technology proposed in Japanese Patent Application Publication No. 2011-220350 (JP 2011-220350 A).
According to the method disclosed in JP 2011-220350 A, the friction engagement element that is in the engaged state is prevented from slipping due to resonance of the power transmission system, thus making it possible to curb reduction of the durability of the friction engagement element.